The invention relates to a displacement device as defined in the pre-characterizing part of claim 1.
Such a displacement device may be used, for example, in a wafer stepper for making integrated circuits. Very accurate and fast displacements in the X- and Y-directions can be obtained by means of the device. In addition, small displacements in a Z-direction perpendicular to the X- and Y-directions are also possible. The displacements are dependent on the phase and the value of the current through the coils. A so-called Halbach magnet configuration is used in the system of magnets. The magnets in a row of magnets are magnetized here such that the magnetization directions of each pair of mutually adjoining magnets are rotated 90xc2x0 relative to one another. The use of such a magnet configuration leads to a stronger magnetic field at the coil side and accordingly to greater forces for displacing the parts relative to one another. U.S. Pat. No. 5,886,432 shows the use of a number of columns of magnets situated next to one another in accordance with the Halbach principle. The distance between the columns of magnets is equal to the width of a magnet here. Air is accordingly present between the columns.
The displacement possibility in the Z-direction allows a levitation of a first part relative to a second part which is movable relative to the first, which is a major advantage because it is made possible to displace the first part and the second part in the XY-plane relative to one another practically without friction with the aid of means which are present anyway. Various solutions are conceivable for the control of such a system, which means need not be fundamentally different from those used in similar systems where no electromagnetic levitation is used but where an air bearing is used for levitation by pneumatic forces, or some other kind of bearing for the displacements along the XY-plane.
The one part will bear on the other part previous to starting of the displacement device if levitation is used, because the levitation forces for separating the two parts from one another are not yet available at that moment. Many existing measuring systems capable of determining a position in an XY-plane operate incrementally, i.e. it is determined from an initial position through counting of steps where the one part is with respect to the other part. The measuring system has lost its position in the case of a power cut, which renders it necessary to find the initial position again upon a restart under the control of the system.
In the case of electromagnetic levitation in accordance with U.S. Pat. No. 5,886,432, moreover, it is necessary to have accurate information on the local positions of the electric coils of the second part relative to the magnets of the first part immediately during starting, because the control of the currents through the various electric coils present for obtaining the necessary levitation is dependent on information about the local positions.
The invention has for its object to provide a displacement device of the kind mentioned in the preamble which is capable of supplying the required accurate local position information also immediately upon starting.
The displacement device is for this purpose characterized in that the displacement device is provided with a number of sensors sensitive to magnetic fields, which sensors supply a signal which is dependent on the local mutual positions of the permanent magnets of the first part relative to the electric coils of the second part in the region where these two parts overlap.
The use of sensors sensitive to magnetic fields renders it possible for the first time to obtain direct information on the local position of the second part relative to the first part, even immediately during starting.
Preferably, the embodiment of the invention as defined in claim 2 is used. It is in fact possible to locate the sensors sensitive to magnetic fields within this part in a manner such that they are interfered with as little as possible by the interfering varying magnetic fields of the electric coils.
Various types of sensors sensitive to magnetic fields are in existence. Preferably, however, Hall sensors are used in accordance with claim 3. These are indeed eminently suitable for the envisaged aim thanks to their property that they are capable of measuring high static magnetic field strengths.
An embodiment of the invention as defined in claim 4 is strongly preferred. The use of one or several arrays of Hall sensors renders it possible to determine the position of the second part accurately within a pole pitch of the periodically recurring field of magnetic poles in the first part by means of suitable electronic circuits. This embodiment also has other advantages, as will be explained in more detail in the description of the Figures.
An embodiment of the displacement device as claimed in claim 5 is furthermore preferred. In this embodiment, the magnetic field is further optimized and the length of the array is defined such that the generated signal is as constant as possible in the case of a displacement of the array in the direction in which the array extends.
In the embodiment of the invention as claimed in claim 6, therefore, the arrays used can be fully identical in view of the occurring symmetries of the magnetic field profiles in both diagonal directions.
The embodiment of the invention as defined in claim 7 is preferably used. This embodiment, which will be explained in more detail in the description of the Figures, renders it possible to determine the position of the second part of the displacement device relative to the first part within a single pole pitch both in the X-direction and in the Y-direction through the use of no more than four identical Hall arrays placed at a distance from one another and through the use of suitable supporting electronics.
The further embodiment of the invention as claimed in claim 8 renders it possible in addition to measure small angular rotations of the second part relative to the first part of the displacement device in a suitable manner in that a difference is measured between the output signals of systems of Hall sensors which lie in one another""s extended direction.
In the embodiment of the invention as claimed in claim 9, it is possible to derive from the amplitudes of the signals of two arrays belonging to one another the vertical distance between these arrays and the magnet plate, i.e. the vertical distance between the two parts of the displacement device.
The embodiment of claim 10 relates to an advantageous positioning of the Hall arrays relative to the electric coils of the second part of the displacement device for minimizing any interference which may be caused by the magnetic field of the electric coils. The positioning in accordance with this embodiment can ensure that the influence of the coils on the Hall sensors of an array are predictable and identical. Suitable compensation measures may accordingly be taken in the control electronics for compensating said influence.
Finally, a further embodiment as claimed in claim 11 was found to be advantageous. Linear arrays of Hall sensors each comprise a series of individual Hall sensors. These are found to have comparatively great differences in sensitivity to magnetic fields in practice, which makes it advantageous to have a possibility of individually adjusting the gain factor for each Hall sensor.